A liquid crystal display device (LCD) makes it possible to serve as a display through regulating transmission of light irradiated from a light source by using a liquid crystal panel composed of a liquid crystal cell and polarizing plates disposed on both sides thereof. As the polarizing plates attached to the liquid crystal cell, an absorption type polarizing plate which is called a light-absorption type dichroic linear polarizing plate is generally used. A polarizing plate in which iodine-containing PVA is protected with triacetyl cellulose (TAC) is widely used.
The absorption type polarizing plate transmits light that is polarized in a direction of transmission axis and absorbs almost all of light that is polarized in a direction perpendicular to the transmission axis. It is pointed out that about 50% of non-polarized light irradiated from a light source device is absorbed by the absorption type polarizing plate, and that light utilization efficiency is lowered. Therefore, in order to use efficiently the polarized light in the direction perpendicular to the transmission axis, a configuration, wherein a reflective polarizer called a brightness enhancement film is used between the light source and the liquid crystal panel, has been investigated. As an example of the reflective polarizer, a polymer film that employs optical interference has been investigated (Patent Literature 1 and others).
On the other hand, regarding the polarizing plate attached to the liquid crystal cell, various kinds of lamination configurations including a reflection display that uses outside light and a transmission display that uses backlight, in which the absorption type polarizing plate and reflection type polarizing plate are used in combination, have also been investigated in accordance with kinds and purposes of light used in the display device. An example of the reflective polarizing plate includes an attempt in which a birefringent dielectric multi-layer film is used.
An example of reflective polarizing polymer films, which employ a birefringent multi-layer structure and have been investigated so far, includes Patent Literatures 2 to 4, for example. A reflective polarizing polymer film described in Patent Literature 3 and others exhibits a certain level of polarization performance by means of: using polyethylene-2,6-naphthalene dicarboxylate (hereinafter, called as 2,6-PEN in some cases) for a high refractive index layer and PEN copolymerized with a thermoplastic elastomer or terephthalic acid in an amount of 30 mol % as a low refractive index layer; increasing the difference in interlayer refractive indexes in a stretching direction (X direction) through stretching so as to enhance reflectance of P polarization; and, on the other hand, decreasing the difference in interlayer refractive indexes in a direction (Y direction) perpendicular to the in-plane X direction of the film so as to enhance transmission of S-polarized light.
However, when the polarization performance is tried to be enhanced to the level of the dichroic linear polarizing plate, difference arises between the refractive index in Y direction and the refractive index in the direction of film thickness (Z direction) while stretch develops in X direction; when the difference in the interlayer refractive indexes in Y direction is accorded, the difference in the interlayer refractive indexes in Z direction becomes large; accordingly, partial reflection of light that enters in an oblique direction makes hue shift of transmitted light larger.
Therefore, a liquid crystal display device that uses the polymer film having the above multi-layer structure by itself as one of the polarizing plates of a liquid crystal cell has not been put into practical use yet.
In addition, regarding a reflective polarizing polymer film that is replaceable for the absorption type polarizing plate and has a multi-layer structure and a higher polarization degree than before, a proposal of using a specific polymer for a high reflective index layer and a capability of using for the polarizing plates disposed adjacent to the liquid crystal cell are described in Patent Literature 5. However, although the reflective polarizing film attains a high polarization degree of around 97% to 98%, a still higher level of polarization performance has not been attained.
On the other hand, Patent Literatures 6 and 7 and others propose a configuration of a hybrid polarizer in which an absorption polarizer or an absorption element is disposed between two uniaxial birefringent reflective polarizers. When the absorption element and others are not used between the two uniaxial birefringent reflective polarizers, among polarized light components that are to be reflected, half of transmitted leaking from the first reflective polarizer leaks also from the second reflective polarizer due to the effect of multiple reflections. In order to reduce the leak of the transmitted light, the absorption element and others are described to be disposed between the two uniaxial birefringent reflective polarizers.
In addition, as an illustrative embodiment of the hybrid polarizer, an embodiment and others are described, in which an alternative laminate is used as a reflective polarizer, wherein the alternative laminate is composed of a uniaxially aligned layer composed of 90% of polyethylene naphthalate and 10% of polyethylene terephthalate and an isotropic layer having low refractive index composed of a blend of polycarbonate and copolymer polyester, and an absorption layer obtained by mixing a dichroic dye with PVA is coated on a PET cast film and is uniaxially aligned is disposed between the two reflective polarizers. Also, in the other illustrative embodiments, an absorption element, formed in advance by coating in a step different from the step in which the reflective polarizers are formed, is disposed between the two reflective polarizers. Thus, a complicated fabrication method has been employed. Furthermore, in these embodiments of the hybrid polarizers, lowering in adhesion between the reflective polarizer and the absorption element is concerned.                Patent Literature 1: Japanese Patent Unexamined Patent Application Publication No. H9-507308        Patent Literature 2: Japanese Patent Application Laid-Open Publication No. H4-268505        Patent Literature 3: Japanese Patent Unexamined Patent Application Publication No. H9-506837        Patent Literature 4: International Publication No. WO 01/47711        Patent Literature 5: Japanese Patent Application Laid-Open Publication No. 2012-13919        Patent Literature 6: U.S. Pat. No. 2008/0151371        Patent Literature 7: U.S. Pat. No. 2011/0043732        